Metal containers, usually called "tin cans", are often used in the food packaging industry. Such containers have a generally cylindrical configuration. The cylindrical portion of the container is constructed from sheet steel, which may or may not be coated with tin. The sheet steel is formed into a cylindrical configuration and the ends of the sheet are joined along a seam. This seam is usually soldered to provide the container with an airtight seal.
The metal used to construct such containers is ordinarily provided with a protective coating before the metal is used to form the container so that the metal itself will not react with the contents of the container. However, the solder used at the seam is exposed after the container has been constructed. Solder includes a major proportion of lead, and chemical or corrosive attack by the contents of the container can result in a substantial introduction of lead into the contents. Where food is involved or other liquid which is sensitive or its subsequent use is sensitive to lead, it is necessary that the solder be completely coated with a coating which is impermeable to the can contents. In addition, as the can is being constructed, the protective coating applied to the original metal may become scratched or otherwise damaged, particularly at the seam, and it may be necessary to recoat the remainder of the container.
To protect the contents of the container from the solder at the seam and other metal exposed as the container is constructed, the usual procedure in the construction of such containers is to apply two full 360.degree. protective coatings to the interior of the container. The containers are spun about their central axes, and a nozzle is introduced along the axis of the container as it is spinning to fully coat the interior of the container.
Two coatings are used in the prior art to insure that the seam of the container, and particularly the soler used at this seam, are fully and completely coated. Relatively heavy coats are applied each time to further insure that the solder is fully coated and no portions of the seam are exposed. However, by using two such heavy coats throughout the entire interior of the can, a substantial amount of such coating material is applied to the interior of the container other than at the seam. These two heavy coatings are unnecessary other than at the seam because only minor scratches in the protective coating already on the metal exist on the interior of the container other than at the seam. As a result, such conventional processes result in a substantial waste of such coating material.